The Eastleigh battle of 2013 will require a chapter of its own in any future analysis of parliamentary by-elections.

The basic ingredients that make a by-election classic are there: former MP resigns seat in disgrace, defending party’s poll ratings looking distinctly bleak.

But when we add the spice of coalition tensions, political defections, party scandals and potential for tactical voting then who knows what the result and more importantly its aftermath will be.

For almost 9,000 Eastleigh voters, the deed is already done.

Image:Nick Clegg campaigning for the Lib Dem candidate Mike Thornton

These are the postal voters that made up their minds last week before the UKIP MEP defected to the Conservatives and before Nick Clegg was engulfed by the allegations facing Lord Rennard.

It is interesting that while 12,644 Eastleigh electors opted for a postal vote prior to the last general election, the figure now stands at 14,267.

So, what, or more pertinently who over the past two years, persuaded these additional electors to switch to a postal vote?

I don’t know the answer to this question but what I do know is that in Eastleigh in 2010 about one in five votes were cast by electors using postal votes.

In such a close race, where Chris Huhne's winning margin was fewer than four thousand votes, the higher turnout among postal voters was probably of critical importance.

I also know that contrary to the defeats inflicted upon Liberal Democrat councillors in recent local elections, those in Eastleigh have bucked the trend - strengthening their grip on a council they have controlled since the late 1980s.

Make no mistake, therefore, it was no fluke when the Liberal Democrats inflicted a huge defeat here on the Conservatives at the by-election in 1994 that followed the death of Stephen Milligan.

And it was no fluke when the Eastleigh Liberal Democrats put in one of their party’s best performances nationally at the 2010 general election.

Their chosen candidate, local councillor Mike Thornton, is already an important cog in a formidable and well-entrenched party machine.

So, the pattern of recent voting suggests that the seat is his for the taking.

Even the betting market agrees, with the Liberal Democrats odds-on to retain the seat.

But it is votes and not the views of the punters (and even pundits) that determine elections.

And in this regard the polling evidence is fascinating.

Two companies, Populus and Survation have conducted polls amongst Eastleigh’s now besieged electors.

The latest Populus survey for The Times, conducted from February 19-22, provides headline figures of Conservative 28% and Liberal Democrats 33%.

As a point of reference, the vote shares in 2010 were 47% for Huhne and 39% for the Conservative Maria Hutchings.

The first point to make is that both front-runners are likely to lose vote shares - the result will be decided by the party whose vote declines the least.

UKIP is the key to this by-election since the Labour vote in Eastleigh is not tracking the party’s national improvement.

If UKIP attracts support from disgruntled Conservatives who find Osborne’s austerity tonic unpalatable, then the Tory challenge is doomed.

On the other hand, if UKIP has truly become the party of protest now that the Liberal Democrats are in government, then it could begin to attract those who voted for Chris Huhne in 2010 because he was neither Labour nor a Conservative.

The second point to make about the polling evidence is that it pays to study the fine detail behind the headline figures.

The latest Survation poll, for example, surveyed a total of 543 Eastleigh residents but then reduced that number to 410 respondents to take account of likely non-voters.

Survation openly report these figures and even provide a margin of error on any vote forecast.

So, that 29/33 split in favour of the Conservatives disappears when we take such matters into consideration.

Equally, the polls currently reporting a narrow Liberal Democrat lead could be forecasting the wrong winner and yet still be correct about estimated vote share when accepted margin of error is accounted for.

We will know the real outcome in the early hours of Friday morning.

But the Eastleigh effect will extend much further, making this truly one of the most important by-elections in the post-war era.